Key Factors in Enhancing Pseudocapacitive Properties of PANI-InOx Hybrid Thin Films Prepared by Sequential Infiltration Synthesis

Sequential infiltration synthesis (SIS) is an emerging vapor-phase synthetic route for the preparation of organic–inorganic composites. Previously, we investigated the potential of polyaniline (PANI)-InOx composite thin films prepared using SIS for application in electrochemical energy storage. In this study, we investigated the effects of the number of InOx SIS cycles on the chemical and electrochemical properties of PANI-InOx thin films via combined characterization using X-ray photoelectron spectroscopy, ultraviolet–visible spectroscopy, Raman spectroscopy, Fourier transform infrared spectroscopy, and cyclic voltammetry. The area-specific capacitance values of PANI-InOx samples prepared with 10, 20, 50, and 100 SIS cycles were 1.1, 0.8, 1.4, and 0.96 mF/cm², respectively. Our result shows that the formation of an enlarged PANI-InOx mixed region directly exposed to the electrolyte is key to enhancing the pseudocapacitive properties of the composite films.


Introduction
Organic-inorganic hybrid materials have received continued attention owing to their novel functionalities, which are not demonstrated in single-phase materials, whether organic or inorganic. In particular, electrically conductive organic-inorganic composite films have demonstrated enhanced properties for application in electrochemistry-related areas. Some recently reported examples of the benefits of using PANI-metal oxide composite films include enhanced efficiency in photovoltaic cells [1], sensitivities and linearities in sensors [2], photocatalytic efficiencies in catalysts [3], and retention properties in supercapacitors [4]. The different properties of hybrid thin films and their performances in different areas depend primarily on the synthesis routes of the thin films. Therefore, the development of novel techniques for preparing organic-inorganic hybrid thin films has received attention in various areas.
Sequential infiltration synthesis (SIS) is a new vacuum-based technique for preparing organic-inorganic composites and is considered a variant of atomic layer deposition (ALD). Although ALD exploits self-limiting chemical reactions on the surfaces, SIS utilizes chemical reactions within the bulk polymer phase [5]. For SIS reactions to occur readily, the precursors designated to be used must first infiltrate the polymer phases. Typically, the entrapment of infiltrated SIS precursors by specific functional groups of the polymer is preferred. Poly(methylmethacrylate) (PMMA) [6] and poly(2-vinylpyridine) (P2VP) [7] are typical types of polymers that have been widely used for SIS because the carbonyl groups in PMMA and pyridine groups in P2VP undergo Lewis acid-base reactions with typical SIS precursors such as trimethylaluminum and titanium tetrachloride (TiCl 4 ). However, the Polymers 2023, 15, 2616 2 of 9 potential applications of hybrid thin films based on PMMA and P2VP are limited to areas where electrical conductivity is not required. Only several studies have been conducted on SIS with conducting polymers and their applications in electrochemistry.
Polyaniline (PANI) is a representative conducting polymer with controllable electrical conductivity owing to doping. PANI can exist in three different chemical states, namely, leucoemeraldine, pernigraniline, and emeraldine, depending on the degrees of oxidation and reduction [8,9]. PANI with an emeraldine base can be transformed into an emeraldine salt, which exhibits high electrical conductivity (~10 2 S/cm) when acid-doped [10]. Wang et al. reported the SIS process of doping PANI with SnCl 4 and MoCl 5 vapors [11], which exhibit the Lewis acidic nature; the doped PANI exhibited a moderate conductivity of 9.8 × 10 −5 S/cm. PANI-ZnO (~18.42 S/cm) [12] and PANI-InO x (4-9 S/cm) [4] composite thin films prepared via SIS showed electrically conductive properties. Previously, we demonstrated the significant potential of PANI-InO x composite films prepared using SIS for electrochemical energy storage, which warrants follow-up studies on the same system [4].
The aim of this study was to investigate the influence of metal oxides on the electrochemical properties of polyaniline-metal oxide composites as energy storage materials utilizing the SIS. Research related to SIS focusing on conducting polymers is limited to several papers, and studies specifically investigating their electrochemical properties are scarce. In this study, we investigated the variations in the chemical and electrochemical properties of PANI-InO x films prepared via SIS as a function of the SIS cycle number. PANI-InO x films exhibit a graded concentration of InO x along the direction of the film thickness, where their structure is determined by the number of cycles. A combination of ultraviolet-visible (UV-vis) spectroscopy, Raman spectroscopy, and attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectroscopy was performed to understand the variation in the chemical structure of PANI in response to alloying with InO x . The superior pseudocapacitive properties of the sample with the optimized cycle number (50 cy) are attributable to the increased volume of the PANI-InO x mixed region, which is exposed to the electrolyte.

Sample Preparation
PANI with an emeraldine base powder (M w~1 0,000, Sigma-Aldrich, Saint Louis, MI, USA) was dissolved in methyl-2-pyrrolidone (≥99%, Sigma-Aldrich, Saint Louis, MI, USA) with a concentration of 30 mg/mL. The solution was stirred for 24 h at 80 • C and 850 rpm. The solution was spun onto prepared substrates at 2000 rpm, and the as-spun substrates were baked at 70 • C in air. The thickness of the prepared PANI thin films was approximately 37 nm. Subsequently, SIS was performed using a thermal ALD reactor (Daeki HighTech, Daejeon, Republic of Korea) with a cross-flow design. The precursors used for the SIS were trimethylindium (TMIn, 99.999%, EasyChem) and H 2 O (99.999%, Sigma-Aldrich, Saint Louis, MI, USA). Ar carrier gas (99.999%) was continuously flowed at 5 sccm during the entire SIS. Both the TMIn and H 2 O half-cycles involved a 1 s dose, 120 s of exposure, and 120 s of purging. The reactor chamber was isolated from the pump during the exposure step to facilitate the infiltration of the precursors into the polymer matrix. The SIS-prepared substrates were annealed at 270 • C for 1 h in the forming gas of H 2 -N 2 (~3.9% H 2 in N 2 ).
Different types of substrates were used for different characterization methods: an Si substrate (n type, 1-10 Ohm·cm, iTASCO) with a 500-nm-thick SiO 2 layer was used for X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. Au-coated Si substrate (Au thickness:~90 nm) was used for ATR-FTIR spectroscopy. A fused silica (iNexus, Inc., Seongnam, Republic of Korea), which had a transmittance of~90% or higher in the wavelength range > 250 nm, was utilized for UV-vis spectroscopy. Electrochemical experiments were performed using glass substrates with a fluorine-doped tin oxide (FTO) layer measuring~600 nm thick (NSG TEC 7, Pilkington, Lathom, UK).

Sample Characterization
HRXPS depth profiling was performed using an X-ray photoelectron spectrometer (Kalpha, Thermo Scientific, Waltham, MA, USA) with Ar + ion beams at 1 kV and an etching time of 10 s. The X-ray source used was monochromatic Al Kα (1487 eV). The surfaces of the annealed samples were partly scratched using stainless-steel tweezers to create a surface step on the sample, and the thickness of the PANI-InO x thin film was measured using a stylus profiler (Alpha-Step ® D-500, KLA, Milpitas, CA, USA). The cleanliness of the scratched surface was confirmed based on spectroscopic ellipsometry data (FS-1, Film Sense, Lincoln, NE, USA) obtained from the surface and a comparison with those of a bare Si substrate. Raman spectroscopy was performed using a Raman spectrometer (LabRAM HR-800, Horiba, Japan) under the following conditions: 514 nm laser source measuring 0.7 µm, 1800 gr/mm grating, 10 s acquisition time, and 10 specular accumulations. UV-vis spectroscopy was performed using a UV-vis spectrometer (UV-2600, Shimadzu, Japan), where an FTIR spectrometer (Vertex 80v, Bruker, Billerica, MA, USA) with a mercurycadmium-telluride detector and a diamond attenuated total reflection (ATR) crystal were used to obtain the ATR-FTIR spectroscopy data at a spectral resolution of 4 cm −1 . An electrochemical analyzer (CHI602E, CH Instruments, Bee Cave, TX, USA) was used to perform cyclic voltammetry (CV) experiments using a three-electrode setup comprising a Ag/AgCl reference electrode, a Pt wire counter electrode, and a PANI-InO x FTO/glass working electrode. CV data were obtained using a pH 7 buffer solution as the electrolyte.

Results and Discussion
We analyzed PANI-InO x composite thin films, which were prepared under different SIS cycles (10, 20, 50, and 100 cy) and annealed in a reducing atmosphere. The sample structure could be summarized as follows: (1) an InO x -rich region, (2) a PANI-InO x mixed region, and (3) a PANI-rich region, as shown in Figure 1a. The thicknesses and chemical compositions of the three regions differed depending on the number of SIS cycles (Figure 1b). The samples with 10 and 20 SIS cycles did not contain an InO x surface layer and only presented a PANI-InO x mixed region and a PANI bulk region. Owing to the repetition of the SIS cycle, the InO x -rich region near the surface of the PANI at times prevented the additional infiltration of the TMIn precursor, thereby resulting in the formation of a thicker InO x layer, which further developed via a mechanism similar to ALD. However, TMIn infiltration in later SIS cycles may not have been completely hindered because the concentration in the PANI-rich region of the 100 cy sample decreased gradually from~40 to 0 at%. Figure 1c shows the oxygen (O) and nitrogen (N) HRXPS data obtained at different locations on the PANI-InO x film, as shown in Figure 1b. The O 1s HRXPS data were deconvoluted into three or four peaks originating from the following components: lattice oxygen (In-O) from InO x at~529.9 eV, oxygen vacancy (V .. o ) at~531.0 eV, indium hydroxide (In-OH) at~532.1 eV, and lattice oxygen (Si-O) from SiO 2 at~533.1 eV [13,14]. In all four samples, the In-O component was the most dominant in the topmost region, which was either an InO x -PANI mixed region (samples with 10 and 20 SIS cy) or InO x -rich regions (Samples with 50 and 100 SIS cy). The components of higher BEs (i.e., −OH and V .. o ) became more dominant compared to the In-O component as the HRXPS analysis region shifted toward the substrate (i.e., PANI-rich region). This is consistent with the previous SIS results, which indicated that the oxidation of the SIS precursors within the polymer matrix was less complete than that on the polymer surface [6,15]. The average stoichiometries of the four samples were InO 0.85 , InO, InO 1.38 and InO 1.44 for 10, 20, 50, and 100 cycles, respectively. The stoichiometry trend was reasonable, considering that the proportion of surface-like InO x compared with that of bulk-like InO x (i.e., synthesized within the polymer matrix) enhanced as the SIS cycle number increased. The N 1s HRXPS data of the four samples were deconvoluted into three components: quinonoid imine (−N=) at ~398.4 eV, benzenoid amine (−NH−) at ~399.5 eV, and protonated amine/imine state (−NH2 + -, =NH + −) at ~400.4 eV [16]. In all the HRXPS spectra, the amine component was more dominant than those of the other components. Meanwhile, the PANI with an emeraldine base usually contained equal amounts of imine and amine components. The presence of protonated species along with a decrease in the number of imine units suggested that the protonated species may have originated from the imine units. PANI doped with HCl contains protonated species transformed from the imine components [17]. However, no clear correlation was indicated between the percentage of protonated species and the InOx content in any sample. Therefore, further studies are necessary to determine the potential chemical reactions contributing to the formation of protonated species during InOx alloying. Figure 2a shows the UV-vis transmittance spectra of the four samples. The samples with 10, 20, and 50 cy showed a weak absorption band at ~610 nm, which was assigned to n-π* transition between the benzenoid and quinonoid rings. The increase in the absorption below ~400 nm from the three sample was likely related to the π-π* transition of the benzenoid ring, which is known to be located at ~320 nm [18]. The 100 cy sample showed stronger absorption in the UV region (<400 nm), whereas the n-π* transition was The N 1s HRXPS data of the four samples were deconvoluted into three components: quinonoid imine (−N=) at~398.4 eV, benzenoid amine (−NH−) at~399.5 eV, and protonated amine/imine state (−NH 2 + -, =NH + −) at~400.4 eV [16]. In all the HRXPS spectra, the amine component was more dominant than those of the other components. Meanwhile, the PANI with an emeraldine base usually contained equal amounts of imine and amine components. The presence of protonated species along with a decrease in the number of imine units suggested that the protonated species may have originated from the imine units. PANI doped with HCl contains protonated species transformed from the imine components [17]. However, no clear correlation was indicated between the percentage of protonated species and the InO x content in any sample. Therefore, further studies are necessary to determine the potential chemical reactions contributing to the formation of protonated species during InO x alloying. Figure 2a shows the UV-vis transmittance spectra of the four samples. The samples with 10, 20, and 50 cy showed a weak absorption band at~610 nm, which was assigned to n-π* transition between the benzenoid and quinonoid rings. The increase in the absorption below~400 nm from the three sample was likely related to the π-π* transition of the benzenoid ring, which is known to be located at~320 nm [18]. The 100 cy sample showed stronger absorption in the UV region (<400 nm), whereas the n-π* transition was primarily suppressed. The absorption in the UV region was likely due to absorption by the thick InO x layer. The Tauc plot of the 100 cy SIS shows that the bandgap of InO x was~3.5 eV, which was slightly lower level than that of the dense InO x thin films reported in the literature [19,20]. The smaller bandgap of InO x along with the presence of tail states was reasonable, considering that a significant portion of the InO x phase was present within the polymer matrix along with a high concentration of oxygen vacancies. The optical bandgap of PANI-InO x samples tends to decrease as the number of SIS cycles decreases ( Figure S1). The ATR-FTIR spectra of the four samples (Figure 2b) showed IR bands related to the PANI phase: (1) stretching of the quinonoid ring at~1600 cm −1 , (2) stretching of the benzenoid ring at~1512 cm −1 , (3) stretching of C-N of the secondary aromatic amine at 1300 cm −1 , and (4) out-of-plane C-H deformation of the 1,4-distributed aromatic ring at 823 cm −1 [21][22][23]. The 100 cy samples showed significant IR bands, with lower intensities compared with the other samples owing to the presence of the InO x surface layer. Similarly, the Raman spectra of the four samples (Figure 2c) exhibited significant Raman bands associated with the PANI phase, as summarized in Table 1. The presence of the C-N +• stretching (radical cations) band at~1350 cm −1 in all samples is consistent with the presence of protonated amine/imine species indicated in the HRXPS analysis. Radical cation bands are typically observed in acid-doped PANI, which suggests that alloying with InO x may offer similar effects on the acid doping of PANI.  The CV results for the samples, measured at a scan rate of 10 mV/s, are shown in Figure 3. All the samples showed a pair of redox peaks at similar potentials (i.e.,~0.2 V vs. Ag/AgCl and~−0.05 V vs. Ag/AgCl). The 50 cy sample indicated better-defined redox peaks with a higher current compared with the other samples. The area-specific capacitance values of PANI-InO x samples prepared with 10, 20, 50, and 100 SIS cycles were 1.1, 0.8, 1.4, and 0.96 mF/cm 2 , respectively. A detailed explanation of this calculation is reported in the literature [4]. The CV measurements were performed multiple times using different samples prepared under the same SIS conditions. The redox peak positions of the CV curves varied slightly within a~100 mV range. Therefore, the subtle variation in the peak position observed for the different SIS samples (10,20,50, and 100 cy) is considered to be within the experimental error. CV curves collected at different scan rates are provided in Supplementary Materials. In order to investigate the capacitance stability of the sample after prolonged exposure to the electrolyte, we conducted a CV experiment consisting of 1000 cycles ( Figure S2). This experiment assessed the evolution of the capacitance over time in response to extended electrolyte exposure. In the electrochemical impedance spectroscopy (EIS) test conducted on the PANI-InO x 50 cycle, pure PANI, and pure InO x samples in a previous study, semicircles were observed at high frequencies and straight lines were observed at low frequencies [4]. Among the three samples, the composite sample exhibited the smallest semicircle, indicating a lower charge transfer resistance. Polymers 2023, 15, x FOR PEER REVIEW 6 of 9  Table 1.
Further analysis is necessary to identify the redox reactions contributing to the observed CV peaks. However, the conversion of the emeraldine and pernigraniline states was speculated to be the primary redox reaction in the PANI-InOx samples in our previous study. The enhanced peak current of the 50 cy sample might have been related to the larger thickness of the PANI-InOx mixed region (Figure 1b) compared with those of the 10  Table 1. and 20 cy samples. The 100 cy sample exhibited a sufficiently thick PANI-InOx mixed region; however, the InOx surface layer likely prevented direct contact between the mixed region and the electrolyte [4]. Furthermore, the 50 cy sample had a larger proportion of protonated amine/imine structures (Figure 1c), a fact which is likely related to the PANI-InOx mixed region.

Conclusions
It is only recently that SIS was proved to be a promising route for the preparation of organic-inorganic hybrid films for electrochemistry and electrochemical energy storage. The goal of this study was to provide a more comprehensive understanding of the effects of the number of SIS cycles (10,20,50, and 100 cycles) on the chemical and electrochemical properties of PANI-InOx composite films. The PANI-InOx films showed a graded composition of InOx within the PANI matrix, with ample concentrations of oxygen vacancies and hydroxide components. The entire film structure was composed of two or three components, including an InOx-rich region, a PANI-InOx mixed region, and a PANI-rich region, and its structure depended on the number of cycles. Combined characterization using HRXPS, UV-vis, Raman, and FTIR spectroscopy consistently revealed the presence of cationic radicals, which might have been related to the transition from the quinonoid structure to the benzenoid structure. The 50 cy samples showed the highest pseudocapacitance among the tested samples, which was likely due to the relatively thick electrochemically active PANI-InOx region and the exposure of the PANI-InOx region to the electrolyte.

Data Availability Statement:
The data presented in this study are available in the article.

Conflicts of Interest:
The authors declare no conflict of interest. Further analysis is necessary to identify the redox reactions contributing to the observed CV peaks. However, the conversion of the emeraldine and pernigraniline states was speculated to be the primary redox reaction in the PANI-InO x samples in our previous study. The enhanced peak current of the 50 cy sample might have been related to the larger thickness of the PANI-InO x mixed region ( Figure 1b) compared with those of the 10 and 20 cy samples. The 100 cy sample exhibited a sufficiently thick PANI-InO x mixed region; however, the InO x surface layer likely prevented direct contact between the mixed region and the electrolyte [4]. Furthermore, the 50 cy sample had a larger proportion of protonated amine/imine structures (Figure 1c), a fact which is likely related to the PANI-InO x mixed region.

Conclusions
It is only recently that SIS was proved to be a promising route for the preparation of organic-inorganic hybrid films for electrochemistry and electrochemical energy storage. The goal of this study was to provide a more comprehensive understanding of the effects of the number of SIS cycles (10,20,50, and 100 cycles) on the chemical and electrochemical properties of PANI-InO x composite films. The PANI-InO x films showed a graded composition of InO x within the PANI matrix, with ample concentrations of oxygen vacancies and hydroxide components. The entire film structure was composed of two or three components, including an InO x -rich region, a PANI-InO x mixed region, and a PANI-rich region, and its structure depended on the number of cycles. Combined characterization using HRXPS, UV-vis, Raman, and FTIR spectroscopy consistently revealed the presence of cationic radicals, which might have been related to the transition from the quinonoid structure to the benzenoid structure. The 50 cy samples showed the highest pseudocapacitance among the tested samples, which was likely due to the relatively thick electrochemically active PANI-InO x region and the exposure of the PANI-InO x region to the electrolyte.